def printGrade(score):
    if score >= 90.0:
        print("a")
    elif score >= 80.0:
        print('b')
    elif score >= 70.0:
        print("c")
    elif score >= 60.0:
        print('d')
    else: 
        print("f")

def main():
    score = eval(input("请输入分数："))
    print("这个分数的等级为：",end="")
    print(printGrade(score))

main()